1. Field of Endeavor
The present invention relates to the field of methods for the manufacturing and the service of components in the hot gas path of, for example, gas turbines. Specifically, it relates to a method of improved localized build-up of thermal barrier coatings (TBC) on hot gas path parts in gas turbines and other heat engines combined with a comprehensive approach of inspection to better assure the durability of the coating.
2. Brief Description of the Related Art
Coating systems for hot gas path (HGP) parts of gas turbine engines for the protection of components are well known. Many of these coating systems consist of a metallic bond coat (BC) layer and a ceramic thermal barrier coating (TBC) top layer. The TBC layer is predominantly applied to protect the base material of the components against high temperature environments, whereas the metallic BC ensures a good bonding of the TBC layer, but also protects the base material against oxidation and corrosion. During operation the BC/TBC system has to sustain thermal cycling and harsh environmental conditions. Also to be considered are damages due to transport and installation as well as insufficient quality of the coating as produced in the workshop. As a result, localized loss of the TBC layer can occur, e.g., due to foreign object impacts, phase changes, and fatigue, but also sintering of the ceramic and erosive wear, particularly on highly loaded locations of components. Additionally, in certain cases localized uncoated areas on new manufactured components have to be subsequently TBC coated in a flexible and easy manner. Consequently, there is a need to perform local application as well as local repair of TBC layers to allow further operation.
Local application (local initial application as well as local repair of local damages) of TBC with a thermal spray technique, as, for example, disclosed in U.S. Patent Application Publication No. 2007/0063351 A1 or U.S. Pat. No. 5,972,424, similar to the technique used to apply TBC on new manufactured parts (see, e.g., U.S. Pat. Nos. 4,248,940 and 3,006,782) has some advantages. A satisfying adhesion of the repaired coating, a controlled microstructure and phase are for example known to be provided by such a local application process. However, thermal spray techniques are more suitable, e.g., for a local application off-site in dedicated sites for manufacturing and repair than for on-site use. Health and safety issues, cost and technology status of portable devices are boundary conditions, which prevent the use of spray techniques for local application such as repair on-site. Further disadvantages are the accessibility of the components when mounted in the engine and contamination of the hot engine parts in the vicinity of the local application spot due to the local application process.
In comparison, wet application seems more suited and has many advantages in terms of, e.g., costs and easy processing. Such local application of TBC with wet processing, like, for example, using slurry or sol-gel methods, have been investigated many times in the past already. One challenge is to coat a layer with an adapted and sufficient thickness, which is at least equivalent to the one of the original TBC. Sol-gel techniques, as for example described in U.S. Pat. No. 6,235,352, ensure a good bonding of the newly constituted layer but lead generally to an insufficient layer thickness. Another relevant concern by using wet chemical processing is that during drying and curing the applied layer has a pronounced tendency to shrink leading to cracks, bonding defects and spallation.
Attempts to increase the layer thickness, reduce shrinking, and prevent cracking have been pursued in the state-of-the-art, e.g., by adding oxide particle fillers in the sol-gel solution or to the slurry as, for example, disclosed in U.S. Pat. No. 5,585,136 and U.S. Patent Application Publication No. 2007/0224359 A1. Similarly, hollow spheres were suggested to serve as filler material, for example in U.S. Pat. No. 5,759,932.
Another issue with the wet chemical processing is to achieve a suitable viscosity in order to coat parts with a complex geometry or in order to coat parts mounted inside the engine (in particular if the surface to be treated is in a vertical position or is facing downwards). In this context, EP 1 739 204 proposes a composition for the slurry having an optimal thixotropic behavior. Another approach is disclosed in EP 1 806 423, in which UV curable polymers are used in order to provide a rigid polymer matrix.
U.S. Pat. No. 5,972,424, proposes a method to repair a gas turbine engine component coated with a thermal barrier coating that includes a metallic bond coat and a ceramic top coat by removing the complete ceramic top coat and parts of the metallic bond coat from an engine-run gas turbine engine component and by inspecting the component. After an inspection step, a metallic flash coat is applied to at least a portion of the component. A ceramic top coat is then applied over predetermined portions of the component, including the portion to which the metallic flash coat was applied.
U.S. Patent Application Publication No. 2007/202269A1 proposes local repair of a thermal barrier coating system on a turbine component that has suffered localized spallation wherein the proposed process includes locally cleaning a spalled region with water to remove the remaining coating from the spalled region and to form a tapered profile in the existing thermal barrier coating; and locally thermally spraying a powder mixture into the cleaned localized spalled region to form a repaired thermal barrier coating. The repaired thermal barrier coating system is integrated with the tapered profile to form a seam free of gaps.
The main problems associated with the repair or local application processes according to the state-of-the-art are as follows. In some cases the complete TBC coating is removed from the component and re-applied (see, e.g., the aforementioned U.S. Pat. No. 5,972,424) rather than keeping the defect-free part of the coating and removing only degraded areas. This is a costly and time consuming process.
Furthermore, a comprehensive inspection for different defect types is not considered in the prior art. Particularly, it is missing that inspection has to be performed prior to repair with appropriate tools in order to locate all degraded areas of the BC/TBC system and in order to only locally repair where it is necessary and appropriate. For example, it is not sufficient just to clean regions with spalled-off TBC, as described for instance in U.S. Patent Application Publication No. 2007/0202269 A1. Different defects will be overlooked in such an approach.
In view of the above, the disadvantages/limits in the state-of-the-art as concerns repair can be summarized as follows. Comprehensive inspection is not considered for the whole component, and for all types of degradation such as TBC erosion, cracking, spallation, delamination, sintering, consumption, oxidation, and corrosion of bond coating (BC) and base metal (BM). Inspection during a repair procedure (intermediate inspection in case the coating consists of several layers) is not considered, and in most of the cases the BC/TBC coating system is completely stripped after service and recoated rather than to inspect it and derive a lifetime statement of the remaining coating and to repair only degraded TBC regions. A final inspection step after the coating application is not considered. Further the reachable layer thickness by pure wet application methods is in general limited and usually a high shrinkage of the applied coating leads to macrocracking as well as weak bonding of the coating to the substrate due to the shrinkage, and the strain tolerance of the suggested coating systems is in general not sufficient. Usually, the thermal barrier effect of the applied coating is not sufficient, complex shapes (convex/concave) are difficult if not impossible to repair with approaches mentioned in prior art, and the same is valid for coating application in a vertical position of the component. The stability of the wet applied coatings against high temperature and repeated temperature changes (thermal cycling) in general not sufficient.